Functional optical devices and methods for producing them

ABSTRACT

A functional optical device has cores which are trenches, different portions of the cores being formed from different core materials. The optical device can be formed by forming trenches  5,7,9  within a substrate (normally a substrate  1  covered by a cladding layer  3 ), covering at least part of at least one trench  7  with a cover  11 , depositing a first cladding material to fill the trenches  5,9  which are not covered, removing the cover  11 , depositing a second cladding layer  15  of a second cladding material to fill the trenches  7  which were previously covered, removing core material outside the trenches  5,7,9  and applying a cladding layer to cover the trenches.

FIELD OF THE INVENTION

The present invention relates to optical devices of the kind whichtransform light transmitted through them (“functional optical devices”).The invention further relates to methods for producing the functionaloptical devices.

BACKGROUND OF INVENTION

Recently there has been a growth in demand for improved functionaloptical devices, especially for use in DWDM (dense wavelength divisionmultiplexing) systems. It has become necessary to provide improvedfunctional optical devices such as Mux (multiplexer) devices, DeMux(demultiplexer) devices, amplifiers, optical switches and VOA (variableoptical attenuator) devices.

It is known to form such devices as PLC (planar lightwave circuits), inwhich light moves along paths defined by cores extending over asubstrate in a plane parallel to the surface of the substrate.Conventional PLC fabrication methods produce cores which are eitherridge structures or trench structures. Ridge-type cores are ridges of aselected core material upstanding from a layer of cladding materialformed over the substrate. Trench-type cores are formed by depositing aselected material within a pre-formed trench in a layer of claddingmaterial which itself overlies the substrate. Generally, ridgestructures are more common than trench structures.

The fabrication method of a ridge-type core is typically as follows.Firstly, a substrate such as Si is covered by a cladding layer of acladding material (such as SiO₂), and then a layer of core material(such as SiO₂ including GeO₂). which is to be formed into the core. Amask is applied in selected regions over the core layer, and the exposedportions of the core layer are etched by an etching process such asreactive ion etching (RIE), to leave the ridges of the core material.After the mask is removed, a further cladding layer is provided over thesurface of the device, so that the ridges are fully embedded between twocladding layers.

Although this method is successful, it is difficult to modify it toproduce a device in which different portions of the core are formed ofdifferent materials, since depositing each core material requires aseries of process steps. Additionally, it is difficult to control thethickness of the two different core materials at regions when they meet.

One example of a device having two different core materials is U.S. Pat.No. 6,201,918, which describes a device in which a Mach-Zehnderinterferometer having two optical fiber arms is processed by splicing anoptical path changing segment into one of the arms.

SUMMARY OF THE INVENTION

The present invention aims to provide new and useful optical devices,and new and useful methods for producing optical devices.

In general terms the present invention proposes that an optical deviceis formed having one or more cores which are trenches, differentportions of the core(s) being formed from different materials.

The invention is based on the realisation that it is easier to formtrenches of different materials than to form ridges of differentmaterials. This factor more than compensates for the factors because ofwhich ridge-type cores are normally preferred to trench-type cores.

The optical devices may be formed by the steps of forming trencheswithin a cladding layer (normally a cladding layer which is located on asubstrate), covering certain areas of the trenches, depositing a firstcore layer of a first core material to fill the trenches which are notcovered, removing the cover, depositing a second core layer of a secondcore material to fill the trenches which were previously covered,removing core material outside the trenches, and applying a claddinglayer to cover the trenches.

Preferably, the core material outside the trenches is removed bypolishing. Previously polishing techniques were not capable of polishingthe whole surface of an optical device with an accuracy on the level ofthe dimensions of desired trenches (e.g. 6 micrometers), which is onereason why ridge-type cores are conventionally preferred to trench-typecores. However, the present inventors observe that advances in polishingtechniques have removed this factor, making trench-type cores moreacceptable.

It is to be understood that the present invention is not limited to thecase in which there are exactly two different core materials. Rather,the present invention makes it possible to form optical devices in whichany number of different materials are used to form different portions ofthe cores. Each material is deposited into the respective portion(s) ofthe trenches at a time when all the other portions of the trenches areeither already filled by a previously deposited core material orcovered.

The present invention makes it possible to form a variety of deviceshaving cores composed of different materials. Examples of such devicesare given below, and include, but are not limited to amplifier devices,interferometer devices such as Mach-Zehnder interferometers,arrayed-waveguide gratings, thermo-optic switches, variable opticalattenuators, and gain flattening devices.

In particular the present invention makes it possible to provide opticaldevices which have predefined thermal characteristics.

For example, in some optical devices according to the invention, opticalpaths of differing geometrical lengths include portions of differentrespective core materials (so that they have different optical pathlengths, i.e. the product of the geometrical length and the refractiveindex value). The various core materials are selected to have differentthermal properties, such that although the paths have differentgeometrical lengths, the optical path lengths vary with temperature inthe same way (i.e. the differing core materials compensate for thediffering geometrical lengths of the optical paths). In this way it ispossible to ensure that the performance of the overall optical device isnot temperature dependent.

Alternatively, in other devices a temperature dependence is actuallydesirable. The present invention makes it possible to tailor thistemperature dependence by appropriate selection of different corematerials.

Specifically, a first expression of the invention is a method ofproducing an optical device, the method including the steps of:

-   -   forming trenches within a cladding layer;    -   covering certain areas of the trenches;    -   depositing a first core layer of a first core material to fill        the trenches which are not covered;    -   removing the cover;    -   depositing a second core layer of a second core material to fill        the trenches which were previously covered;    -   removing core material outside the trenches; and    -   applying a cladding layer to cover the trenches.

A second expression of the invention is an optical device having one ofmore cores defining one or more optical paths, each core being formed asa trench within a cladding layer, different portions of the core orcores being composed of different core materials.

BRIEF DESCRIPTION OF THE FIGURES

Preferred features of the invention will now be described, for the sakeof illustration only, with reference to the following figures in which:

FIG. 1, which is composed of FIGS. 1(a) to FIG. 1(e) shows steps in theformation of an optical device which is an embodiment of the invention;

FIG. 2 shows a PLC amplifier device which is an embodiment of theinvention;

FIG. 3 shows a Mach-Zehnder interferometer which is an embodiment of theinvention;

FIG. 4 shows an arrayed-waveguide grating which is an embodiment of theinvention;

FIG. 5 shows a thermo-optic switch which is an embodiment of theinvention;

FIG. 6 shows a variable optical attenuator which is an embodiment of theinvention;

FIG. 7 shows another Mach-Zehnder interferometer which is an embodimentof the invention;

FIG. 8, which is composed of FIGS. 8(a) and 8(b), shows the temperaturevariation with time of a known Mach-Zehnder interferometer, and of theone of FIG. 7;

FIG. 9 shows a waveguide device which is a further embodiment of theinvention;

FIG. 10 shows a cross-sectional view of the waveguide device of FIG. 9;

FIG. 11, which is composed of FIG. 11(a) and 11(b) shows experimentalresults of wavelength temperature dependence (dλ/dT) for comparativeexamples of a Mach-Zehnder interferometer, respectively having corecompositions of (a) 10GeO₂-90OSiO₂ and (b) 8GeO₂-5B₂O₃-87SiO₂; and

FIG. 12, which is composed of FIG. 12(a) and 12(b) shows experimentalresults of wavelength temperature dependence (dλ/dT) for Mach-Zehnderinterferometers, both having the first core material of8GeO₂-5B₂O₃-87SiO₂, and the second core material of 10GeO₂-90SiO₂, whichare embodiments of the invention, respectively having geometric lengthsof the second core material of (a) L_(core2)=15 mm and (b) L_(core2)=17mm.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A method of forming an optical device according to the invention isshown in FIGS. 1(a) to 1(e). Below, a method is shown of producing awaveguide with differing core materials, such as one into which havebeen introduced a dopant such as GeO₂ which increases refractive index,and/or a dopant such as B₂O₃ which is effective to reduce thetemperature dependence of the refractive index.

The method employs a substrate layer 1, which may for example be a Siwafer having a diameter of 3 inches (7.5 cm) and 1 mm thickness.Firstly, the substrate layer 1 (e.g. Si), shown in FIG. 1(a) incross-section, is covered by an under cladding layer 3. The undercladding layer 3 may be a silica glass film, which is formed by plasmaenhanced chemical vapour deposition (referred to here as PECVD). The gasmaterial is tetraethoxysilane (Si(OC₂H₅)₄, referred to here as TEOS) andoxygen (O₂). Trenches 5, 7, 9 within the under cladding layer 3 may beformed using a photolithographic method which is widely used insemiconductor industry. For the silica glass film etching, reactive ionetching (here referred to as RIE) technology is adopted usingfluorine-containing gas. For this, in order to form the trenches with aprecise square form, an inductively coupled plasma (here referred to asICP) RIE apparatus, such as an RIE-200iPC apparatus produced by theSamco company, is useful. As the fluorine containing gas,trifluoromethane (CHF₃) is used at about 5 mTorr. Radio frequencyelectric power of 103 Watts at 13.56 MHz is supplied to the coil. A filmof Cr of thickness 100 nm prepared by sputtering is used as a mask.After the formation of the trenches, the Cr mask remaining on thecladding layer 3 is removed by oxygen plasma etching using the same RIEapparatus.

Alternatively, the substrate itself can be used as the under claddinglayer when the substrate material is applicable for cladding (e.g. if itis silica). These trenches 5, 7, 9 are shown in FIG. 1(a) in an end-onview, and with a square cross-section. Typically, the trenches may havea depth of about 6 micrometers and a width of about 6 micrometers,although of course other dimensions are possible.

As shown in FIG. 1(b) a cover 11 is deposited over a portion of theunder cladding layer 3 by known techniques, such as a lift off processusing a photoresist material as the cover. The cover 11 covers one ormore portions of one or more of the trenches (in FIG. 1(b) it is showncovering trench 7) but exposes other portions of the trenches (in FIG.1(b) it is shown exposing the trenches 5, 9). A first core layer 13 of afirst core material is then deposited, filling the exposed trenches 5,9. The core may be a Ge-B co-doped silica glass film deposited using thePECVD technique. In order to get high quality core glass, an ICP CVDapparatus, such as a PD-160iP apparatus produced by the Samco company,is useful. Any of the following may be used as the raw material gas:TEOS, tetramethoxygermane (Ge(OCH₃)₄, here referred to as TMOG) andtriethoxyborane (B(OC₂H₅)₃). Instead of triethoxyborane,trimethoxyborane (B(OCH₃)₃) may be used as the boron-containing rawmaterial gas. By controlling the flow rate of the raw material gas, theamount of dopant contained in the grown film varies. In order to obtainthe desired composition of the deposited glass film, it is advantageousto adjust the CVD conditions, such as the gas pressure in the vacuumchamber and ICP power. The gas pressure during the deposition processmay be 5.0 pa. The radio frequency power at 13.56 MHz supplied to theICP device and to the substrate electrode may respectively be set to 900and 300W. The substrate temperature may be 250° C. By controlling theflow rate of the raw material gas, a film can be obtained having acomposition of germanium oxide 12.5 mole %, boron oxide (B₂O₃) 6.2 mole%, and silicon oxide 81.3 mole %. The deposition time was 120 minutes,and the obtained film thickness was 7 μm.

As shown in FIG. 1(c) the cover 11 is then removed (again by any knowntechnique, such as an O₂ plasma etching method), thus exposing thetrench 7.

A second core layer 15 of a second core material is then deposited,filling the exposed trench 7, as shown in FIG. 1(d).

As shown in FIG. 1(e), the portions of the core layers 13, 15 which areoutside the trenches 5, 7, 9 are removed, by polishing, leaving cores inthe trenches 5, 7, 9. Specifically, trenches 5, 9 are filled with thefirst core material 13, while trench 7 is filled with the second corematerial 15. An upper cladding layer (not shown) may then be depositedover the surface of the device, so as to cover all the cores.

Note that the polishing should be performed with a high degree ofaccuracy. This is because if, alternatively, the polishing is unevensuch that one side of the substrate surface is polished by a fewmicrometers more than the other side, then the cores on the first sideof the substrate may be partially removed. The remaining trenches willthen be of different depths.

Note that the deposition of the various materials can be accomplished invarious ways according to any known technique(s). For example, any oneof the layers may be formed by chemical vapour deposition (CVD), oralternatively by flame hydrolysis deposition (FHD) employing one of thereactions:SiCl₄+2H₂+O₂-<SiO₂+4HCl,GeCl₄+2H₂+O₂->GeO₂+4HCl, orSi(OC₂H₅)₄+H₂O->SiO₂+organic compounds.

Apart from these materials, materials such as tantalum oxide, titaniumoxide, silicon nitride, tantalum nitride, silicon carbide, tantalumcarbide, titanium carbide can be used.

The method according to the invention for forming waveguides havingdiffering core materials can be used to produce various functionaloptical devices, such as amplifier devices, devices with a wavelengthdivision multiplexing function, and light beam spot-size converters. Inthe following text, preferred examples of such devices are given. Note,however, that the present invention is not limited to these devices,which are presented for the purposes of illustration.

A first device which is an embodiment of the invention is shown in FIG.2, which is a top view of an erbium doped waveguide amplifier (EDWA)device 20 according to the invention. As in known devices, the device 20has an entry portion 21 for receiving a signal (from the left) which isconcentrated into an amplification region 23 in which laseramplification occurs, to generate an output signal in region 25.Excitation light is input through inputs 27, 29 to amplification region23 by couplers 22, 24. The present invention makes it possible to formthe cores in the amplification region 23 of an erbium doped material,while the cores in the other regions 21, 22, 24, 25, 27, 29 are notdoped. This is advantageous because it means that losses in regions 21,22, 24, 25, 27, 29 are reduced.

An example of this embodiment was prepared using FHD (flame hydrolysisdeposition). After fabricating the trenches by RIE, a first corematerial for the regions 21, 22, 24, 25, 27, 29 is formed by depositingGe-doped silica glass soot on the cladding layer to fill up the trench,and consolidating it into a transparent glass film. The startingmaterials in this case were silicon tetrachloride (SiCl₄) and germaniumtetrachloride (GeCl₄). Another possibility would be to use a differentdopant which increases the refractive index, such as phosphorous whichcan be obtained from phosphorous oxy-trichloride (POCl₃).

The second (erbium doped) core material can be obtained by performing asoot deposition step and then following it with an erbium doping step inwhich the soot is subjected to a solution soaking method using anaqueous solution of Erbium trichloride (ErCl₃). We can level of theerbium doping by adjusting the concentration of erbium aqueous solution,and the soaking time to the solution.

A second device which is an embodiment of the invention is shown in FIG.3 which shows a top view of a Mach-Zehnder interferometer 30 having twooptical paths 31, 32 defined by respective cores. Light which is inputto one of the optical paths at the left of FIG. 3 is partiallytransmitted to the other of the optical paths at the coupler 33, andlight on the two paths interacts at the coupler 34. The couplers 33, 34may for example be directional couplers. Between these sections, the twolight paths have respective geometrical lengths L₁ and L₂, which aredifferent. As is well known to an expert, this means that the lightwhich will be transmitted by the device has a wavelength λ which isequal to the refractive index multiplied by ΔL defined as L₁-L₂ anddivided by an integer. The optical path lengths of each of the two pathsare determined by the product of n×L where n is the refractive index ofthe path and L is its geometrical length.

In this embodiment all the cores are formed of the same material, exceptthat the part of the cores in the region marked 35 are formed of adifferent material. Thus, each of the paths has a different length butalso a different refractive index (which we can write respectively as n₁and n₂). The two refractive indexes can be chosen in dependence on thelengths L₁ and L₂ such that, although each of the refractive indexesvaries with temperature (with either a positive or a negative Δn/ΔT),the output of the interferometer is independent of the temperature,since the effects of the varying refractive indices are cancelledbetween the paths. Thus, a temperature independent Mach-Zehnderinterferometer can be achieved.

In this embodiment, cancellation of the effects of the varyingrefractive indices is important. It is obvious that cancellation isobtained even if the region marked 35 is positioned on the opposite arm,namely the shorter arm. The following example is shown for this case.

We have constructed such a device by the following steps. First,trenches with selected dimensions were directly formed within Cr masked(3000 Å) silica substrate (Asahi, AQ, 1 mm thickness) by selectiveplasma etching (Samco, RIE-200iPC). CHF₃ and C₃F₈ were used as etchinggas at a process pressure of 0.4 Pa. The Cr mask was then removed by O₂plasma etching using the same system. A Ge—SiO₂ or Ge—B—SiO₂ film wasdeposited by an inductively coupled plasma chemical vapor deposition(ICP-CVD, Samco PD-160iP). Tetraethoxysilane, tetramethoxygermane, andtriethoxyborane were used as the source materials for SiO₂, GeO₂, andB₂O₃ growth, respectively. The trench gap filling results were observedby SEM. Component analysis and depth profile study were applied usingTOF-SIMS. The refractive indexes of the films were checked by a prismcoupler. We could control the refractive indexes by adjusting the Si,Ge, and B content in the film. The sample was then annealed at 1000° C.in an atmosphere for 2 hrs after deposition to stabilize the refractiveindex of the core materials. The core materials outside the trenches areremoved by planar surface polishing (Nanofactor, NF300) with accuracy of0.2 μm. The trenches were over-cladded using another silica glasssubstrate by perfect optical contact followed by 1000° C. thermalbonding. The polished surface is smooth enough for this cladding methodto be applied. As a result, buried waveguides with symmetrical structurewhich might reduce the polarization dependent loss were obtained in veryshort time and with low cost.

As discussed above, certain areas of the trenches were covered duringthe first core material deposition. The covering could be performed by alift off process using certain material as a photoresist. However, wewere short of good photoresist material, so instead we used sharp-edgedglass chip with selected length. The trenches that were not covered werefilled with the first kind of material by the ICP-CVD process. The coverwas then removed. The second core material was deposited by the ICP-CVDprocess to fill the trenches which were previously covered. After anannealing process, the samples were polished to remove the films outsidethe trenches and cladding material was applied to the surface by opticalcontact and thermal bonding.

The Mach-Zehnder interferometer filter was designed as shown in FIG. 3(and employing geometric principles used in U.S. Pat. No. 6,201,918),with double directional 3 dB couplers at 1.55 μm range. The coredimension is 7.5 μm and the corresponding refractive index of the corewas 1.4632 at 0.6328 μm. The two light paths had a geometric lengthdifference of ΔL≈1 mm, resulting in a pass-band pitch of 200 GHz (1.6nm).

As a comparison, Mach-Zehnder interferometer devices were made usingonly one kind of core material. Silica based materials with differentdoping ratios of Ge (8-10 mol. %) and B (1-5 mol. %) were used to testwavelength temperature dependence (dλ/dT) for the device with one corematerial. Then, embodiments with two kinds of core material wereprepared to test the dλ/dT characteristics.

The device characteristics were measured using a tunable laser source(Agilent 81689 A) with a wavelength range from 1.525 to 1.575 μm, and apower sensor (Agilent 81634 A). The temperature of the Mach-Zehnderinterferometer filters was controlled to be −20, 0, 30, 51 and 80° C. bya temperature chamber (Yamato IW 241) during measurement. The wavelengthtemperature dependence (dλ/dT) of the filter device was calculated asthe pass-band peak shift against temperature change.

The wavelength temperature dependence (dλ/dT) of the Mach-Zehnderinterferometer with one core material at the temperature range from −20°C. to 80° C. is shown in Table 1. As shown the GeO₂ and B₂O₃ doping ofthe core materials effectively changes the refractive index and dλ/dT;our data also shows that the refractive index decreases from 1.4640 to1.4632 and dλ/dT decreases from 9.5 pm/° C. to 8.1 pm/° C. when the B₂O₃concentration increases from 0 to 5 mol. % while the GeO₂ concentrationremains at 8 mol. %. The lowest optical propagation loss of ˜0.1 dB/cmat 1550 nm of our Mach-Zehnder interferometer devices were obtained. Thehigher propagation loss of 1.53 dB/cm might be caused by particlecontamination in the waveguides during the fabrication process, whichcan be reduced by applying a sample cleaning technology in our process.The trench type Ge—B—SiO₂ planar waveguides exhibit reasonably low lossfor the wavelengths of interest in integrated optics, and thus havepromising applications. TABLE 1 Propagation Core composition n (@632.8nm) dλ/dT@1550 nm loss @1550 10GeO₂—90SiO₂ 1.4652 9.7 0.23 8GeO₂—92SiO₂1.4640 9.5 0.12 8GeO₂—1B₂O₃— 1.4640 9.4 0.18 91SiO₂ 8GeO₂—2B₂O₃— 1.46389.2 0.54 90SiO₂ 8GeO₂—4B₂O₃— 1.4635 8.9 1.53 88SiO₂ 8GeO₂—5B₂O₃— 1.46328.1 0.11 87SiO₂

Due to the results summarized in Table 1, the two compositions of8GeO₂-5B₂O₃-87SiO₂ and 10GeO₂-90SiO₂ were chosen to be core material 1and core material 2, respectively, to prepare the Mach-Zehnderinterferometer filters of FIG. 3 (but with the region 35 positioned onthe path 32, i.e. on the opposite shorter arm) by the multi-corefabrication method described above to test the athermal property. Theyhave significant different dλ/dT of 8.1 pm/° C. and 9.7 pm/° C. as shownrespectively in FIGS. 11(a) and 11(b), which correspond to differentrefractive index temperature dependences of dλ/dT and dn₂/dT,respectively. The reason we chose 8GeO₂-5B₂O₃-87SiO₂ as core material 1is that its refractive index 1.4632 is close to the designed refractiveindex value at coupling area. For the devices consist of two differentcore material sections with different values of dn₁/dT, dn₂/dT, and acertain relationship between their lengths, the athermal condition is:(dn ₁ /dT)ΔL=[(dn ₂ /dT)−(dn ₁ /dT)]L _(core2).

Here L_(core2) is the geometric length for core material 2(10GeO₂-90SiO₂) section at the shorter path. Although the optical pathlength of each of the core material varies with temperature, the effectsof the varying optical path lengths can be made to cancel by choosing asuitable value for the length L_(core2); thus the output of the deviceis independent of the temperature. Because the substituted region 35 ofthe second core material still has the waveguide structure, we canadjust the geometric length L_(core2) without worrying that it willgenerate extra propagation loss. We tried different L_(core2) from 7.6mm to 17 mm, different values of dλ/dT from 3.75 to −2.85 pm/° C. withtemperature from −20° C. to 80° C. were obtained in our experiments asshown in Table 2. The theoretically estimated values of dλ/dT withdifferent L_(core2) are also listed for comparison, which agree fairlywell with the experimental results. TABLE 2 Geometric length dλ/dT@1550nm (pm/° C.) Propagation loss L_(core2) (mm) Estimated Measured @1550(dB/cm) 7.6 3.89 3.75 0.64 9.6 2.21 2.00 1.77 15 0.75 0.54 1.73 17 −2.93−2.85 0.61

Our best athermal result of the prepared Mach-Zehnder interferometerfilter shows the dλ/dT of 0.5 pm/° C. when L_(core2)≈15 mm, as shown inFIG. 12(a). This is small enough even for the 50 GHz pitch filters. Themeasured 3-dB bandwidth of 1.6 nm is close to the designed channelseparation. By increasing the geometric length L_(core2) to 17 mm, evena negative value of −2.85 pm/° C. for dλ/dT was obtained, as shown inFIG. 12(b)! This shows that it is possible for us to control the dλ/dTfrom positive to negative value for different functional purposes. Theexcess loss can be reduced to be negligible by optimizing thefabrication technology.

A third device which is an embodiment of the invention is shown intop-view in FIG. 4, an AWG device 40. The device has an input region 45and an output region 46, spaced apart by an array region 47 in whicheach of the paths flexes. The device 40 has two coupling sections 48,49, for example slab waveguides. A light signal containing twowavelength components with respective wavelengths λ₁ and λ₂ is launchedinto some path in the input region 45, and transmitted through thedevice obtaining one path of the output region 46 has wavelength λ₁ andanother wavelength λ₂. Most of the cores are composed of a first corematerial, but the portion of the cores which intersect within thesubstantially triangular region 4 are of a second (different) corematerial from the material outside the triangular region 4. Note thatthis means that the different light paths, which are of differentrespective geometrical lengths in the region 4, include differentrespective geometrical lengths of the second core material (i.e. theintersection of the optical paths with the region 4 varies with thegeometrical length of the paths). An appropriate selection of the twocore materials provides temperature compensation of the kind describedabove, whereby the functionality of the AWG is temperature independent.

The arrangement of FIG. 4 is appropriate when Δn/ΔT of the cores in theregion 4 is lower than the Δn/ΔT of the cores outside the region 4. Ifthe opposite were true, the triangle 4 could be formed verticallyinverted (i.e. to point upwards in FIG. 4) so that the longer pathswould have a shorter geometrical length of intersection with the region4.

An array waveguide diffraction grating using the above structure is madeas follows. Using a synthetic quartz glass (SiO₂) substrate of thickness1 mm and diameter 76 mm (3 inches) as a direct cladding, trenches areformed using RIE in an optical waveguide pattern forming an arraywaveguide diffraction grating. The waveguide regions 45, 46, 47 shown inFIG. 4 were formed with a width of 6 μm, and a depth of 6 μm, and thenarrowest spacing (i.e. the width of the cladding formed between twoadjacent channels) is appeared as 4 μm at the region of FIG. 4 in whichthe array waveguide 47 meets the coupling sections 48, 49. The couplingsections are slab waveguides having a width of 5 mm, a length of 12 mmand a depth of 4 μm.

The second core material of the triangular region 4 in FIG. 4, and thefirst core material of the other trenches are formed by PECVD. Annealingprocess after deposition is effective to improve the qualities of theglass, such as suppressing fractuation of the refractive index, andremoving impurities such as hydrogen. The annealing conditions are 1100°C. and 30 minutes. This temperature is higher than the glass transitiontemperature of the core glass, but lower than the softening temperatureof the cladding glass which is SiO₂.

The surface of the embodiment is polished to remove the films outsidethe trenches and cladding material was applied to the surface by opticalcontact and thermal bonding. The thermal bonding was carried out for 30minutes at 1100° C.

The upper cladding layer may alternatively be formed by deposition.

Two patterns of AWG were formed at the same time on a single substrate,so in order to get the AWG devices from this substrate, it was necessaryto be diced into a chip having a AWG circuit. Then, the respectivewaveguides of the input region 45 and output region 46 are connected torespective fibres. Signals are input and output using these fibres. Whenthese fibres are provided, the device is completed.

An AWG device was produced in this way, having a operating wavelengthrange of 1.551 μm band and with a channel spacing of 0.8 nm(corresponding to an interval of 100 GHZ), and having 41 channels. Thecharacteristics of this device were measured by connecting one inputport near the middle of the input region 45 to a tunable laser source,and measuring the light signal output from one output port of the outputregion 46. In order to measure the temperature characteristics, thewhole device was installed in an temperature control chamber, and thetemperature was raised in steps, and once the temperature had stabilizedsufficiently the wavelength of the output light signal was measured. Inthe case of raising the temperature from −20° C. to 80° C., the totalchange of the output wavelength of the AWG device with the structure ofthe embodiment was found to be adequately low. Specifically, it was 0.05nm. Note that according to reported data, the change of the outputwavelength in prior art AWG is 0.012 nm/° C., from which one canestimate that the change between −20° C. and 80° C. was 1.2 nm. Due tothis value, even if the channel spacing is 1.6 nm, precise control ofthe temperature of the device is required for practical applications, incase the change in the output wavelength becomes too high.

Note that U.S. Pat. No. 6,304,687 shows an AWG device in which an arrayof waveguides is interrupted by a triangular resin section whichexhibits a negative Δn/ΔT placed in an array region of an AWG in aconfiguration similar to the positioning of the triangular region 4 inthe array region 47. Although there are no core paths in this region,light is able to propagate within the resin from one side to the other,and different light paths include different lengths of resin. Thus, anappropriate selection of the resin material makes it possible to achievetemperature compensation in a way similar to the embodiment describedabove. However, this device is subject to severe losses, since light isundirected while propagating within the resin triangle. The presentembodiment is not subject to this disadvantage, since all light pathshave light guiding structures (i.e. cores and cladding material) alongsubstantially their entire lengths, albeit of differing materials indifferent locations.

Whereas the embodiments above attempt to achieve temperature independentoperation, other known functional optical devices actually utilize thetemperature dependence. For example, a fourth device which is anembodiment of the invention, a thermo-optic light switch 50, is shown inFIG. 5. This device consists of symmetric Mach-Zehnder interferometer(MZ-I). In this case a resistor 51 has a temperature controlled byexternal leads 52, 54, so that the temperature along the portion shownwithin rectangle 53 can be varied in relation to that along otherportions. This means that the temperature along part of the light path55 can be varied in relation to that along a second light path 56.According to the temperature of the portion 53, light input to one ofthe light paths (say path 55) is transmitted either to the opposite endof the same light path or to the opposite end of the other light path.The resistor 51 is set close to the core to detect the temperaturechange sensitively. In this embodiment, over cladding with thickness of20 μm after polishing is performed by FHD method. And then, the resistor51 is prepared by sputtering of Cr on the cladding layer. There are twodirectional couplers 57, 59, and in these regions it is desirable thatthere is small temperature dependence. By using this MZ-I device, aswitching operation can be achieved by changing the temperature alongthe portion shown within rectangle 53. The present invention thusproposes that a device 50 includes a portion of a different corematerial (one having a high temperature dependence) in the region of theoptical path near the resistor 51 (e.g. the portion shown withinrectangle 53), and material of relatively lower temperature dependencein other places to achieve stable coupling condition against temperaturechange. The core materials may be selected from those shown in Table 1.For example, the material having high temperature dependence can be90SiO₂-10GeO₂, and the one having a relatively lower temperaturedependence may be 87SiO₂-8GeO₂-5B₂O₃. In other variations of theembodiment, the GeO2 may be replaced with materials such as tantalumoxide, titanium oxide, silicon nitride, tantalum nitride, siliconcarbide, tantalum carbide, or titanium carbide.

Note that in the absence of heat generated by the resistor 51 the ΔL ofthe two paths is zero (it is symmetric), in contrast with theMach-Zehnder interferometer of FIG. 3 which is intrinsically asymmetric.

A fifth device which is an embodiment of the invention is shown in FIG.6, and is a variable optical attenuator (VOA) 60. The device includes asingle optical path 61 defined by a trench-type core and having an entryportion 62, an exit portion 63 and between them a portion 64 at whichthe temperature is controlled by a resistor 65 operated by externalleads 66, 67. The core material in the region 64 is thermochromic, andthe core material in regions 62, 63 is not thermochromic. This makes itpossible that the regions 62, 63 of the device should have low losses,as compared to known devices in which the core is entirely formed fromthermochromatic material.

A sixth device 70 which is an embodiment of the invention is shown inFIG. 7. Again it is a Mach-Zehnder interferometer, but in this case itis one designed to be controllable, and thus operates as a VOA device.Specifically, a first path extends between an input 71 and an output 73,while a second path extends between an input 72 and an output 74. Thedifferent paths have different respective geometrical lengths L₁, L₂between couplers 75, 76. The first path is heated in a heat-receivingsection of its length by a resistor 78 controlled by electrodes. Allthese features are known in prior art Mach-Zehnder interferometer-typeVOAs. However, in contrast to known systems the device 70 includes aregion 77 of geometrical length L in which the core of the second pathis of a different material from that of the rest of the device, andspecifically one core material has a refractive index which increases toa great extent with increasing temperature, and the other core materialhas a refractive index which increases less than that of the firstmaterial. For example, the refractive index of the core material in theregion 77 may increase less with increasing temperature compared to thematerial which composes the rest of the cores. Writing the increase inthe refractive index of one path as Δn₁ and the increase of therefractive index of the other path as Δn₂, the critical measure isΔn₁-Δn₂.

To understand the operation of the invention consider firstly what theoperation of the device 70 would be if the region 77 were not present.In this case, since L₁ and L₂ have a geometric length difference ΔL, thedevice 70 will only pass light having a wavelength λ given by mλ=nΔLwhere m is an integer and n is the refractive index. Upon a currentbeing applied to the resistor 78, the temperature of the heat receivingsection of the first path will increase, changing its refractive index,and thus changing the wavelength which is passed, resulting in a changeof the transmitted power, so that the device acts as a VOA.Unfortunately, heat will also conduct in time to the lower path,changing its refractive index in the same way, and thus reducing thedifference between the optical lengths of the paths. To address this,the current applied to the resistor 78 must be raised to increase thetemperature of the upper path. As this cycle continues, the ambienttemperature T_(A) of the device, which is the temperature of the secondpath, rises linearly with time, for example as shown in FIG. 8(a) (inwhich the vertical axis indicates the rise in temperature caused by theresistor 78 during the operation of the device), and the temperature T₁which is required to be applied to the heat receiving section of thefirst path by the resistor 78 rises. This gradually rising temperaturesmay cause the device to overheat, unless a cooling device, such ascooling fins, is used, thus increasing the size and cost of the device.

By contrast, since in the embodiment the region 77 is present, when heatspreads from the first path to the region 77 it changes the refractiveindex in the direction opposite to the change it causes in the firstpath. In other words, depending on the value L, the temperaturedependence of the optical length of the second path varies. It may forexample be zero, or it may be opposite to that of the first path. Thus,heat transmission from the first path to the second path need notprevent the Mach-Zehnder interferometer 70 from working, and it is notnecessary to further increase the temperature of the first path tomaintain the operation of the device 70. Thus, the device may beoperated with the first path remaining at a lower temperature than inthe known devices described above, which in turn means that less heat istransmitted to the first path. The device 70 may thus avoid the need forcooling fins to be present. The temperature dependence of the ambienttemperature T_(A) may thus be as shown in FIG. 8(b). Lines 81, 82, 83,84 show, for four respective increasing values of L, the correspondingtemperature at which the temperature of the heat-receiving section ofthe first path is maintained by the resistor 78. Line 82 is the casethat the L is a length (e.g. 18 mm) such that the resistor 78 shouldmaintain the temperature of the first path at the same valueirrespective of the ambient temperature, while lines 83, 84 show theoperation of the device for two higher values of L, and line 81 showsthe operation of the device for a smaller value of L. Note that theinitial rate of increase of temperature shown in line 81 is lower thanthat of T1 in FIG. 8(a). Gradually, all the curves T_(A), 81, 82, 83 and84 eventually reach constant values as an equilibrium state is reached.

As an example of this embodiment, the Mach-Zehnder interferometer-typeVOA device may be an asymmetric one operating at a wavelength of 1.55μm. The geometrical lengths between the couplers 75, 76 may be around 42mm, and they may have a difference of ΔL=4.24 μm. Both the two corematerials may have a refractive index of n=1.4632, but each may have adifferent temperature dependence, such as Δn₁=9×10⁻⁶ and Δn₂=8.01×10⁻⁶respectively. The first core material may be deposited in the whole ofthe core area, except the region 77. The second core material may bedeposited in the region 77, where the waveguide length may be L=18 mm.We prepares a device having these characteristics, and with a heater 78having a length of 2 mm near to the longer path in FIG. 7. Using thisdevice, when the temperature difference between the heater and theambient temperature was required to be 20° C. in order to adjust intothe desirable output power from the device, we found that the differencein temperature remains constant as shown as line 82 in FIG. 8(b) eventhough the ambient temperature gradually increased.

Note that many variations of the device 70 are possible within the scopeof the invention. In particular, it is not necessary that the region 77is provided on the opposite optical path from the resistor 78. Forexample, the region 77 may actually be the heat-receiving section of thefirst optical path.

We now turn to a further embodiment of the invention. This embodiment ismotivated by the known difficulty of coupling two different opticaldevices. For example, it is often desired to couple two devices whicheach contain an optical path (e.g. two devices each containing anoptical fibre), with the end of one fibre being connected to the end ofthe other fibre. The two fibres may have different respective widths(and therefore different refractive indices, even if the material theyare formed from has the same propagation constant). In such situations atechnique known as TEC (thermally expanded core) is used, in which atomsof a material such as Ge are allowed to diffuse outside of the core atthe end of the fibre of narrower width, to thereby increase itseffective width and thus decrease refractive index at the end, so thatthe sizes and refractive indices of the two fibres are made equal at theinterface between them. However, TEC has a number of drawbacks, one ofwhich is that it is not readily applicable to connecting a waveguide toan array of fibres, since the width of the end surface of the waveguideis too great (typically 1 mm, compared to 125 μm for a single fibre) forGe diffusion to be convenient.

To address this, consider a further embodiment of the invention shown inFIG. 9, which is a device 90 (such as a waveguide) which includes cores91 (for illustration only two are shown, but usually the number of coreswill be greater than 2, e.g. 10) and which is to be coupled to anotheroptical device 92 including one or more fibres 93.

In particular, in order to make the waveguide device small adoptingsmall bending radius of the waveguide patterns, one should attempt tokeep the bending loss from becoming large. For this reason, thewaveguides having large refractive index difference between the core andthe cladding and small size of the core, so-called super-high deltawaveguides, are very useful. In this case, the coupling loss at theconnection between the waveguide and a conventional single mode opticalfibre (with a core diameter of 8 μm) will become very big. This is thetypical difficulty of coupling two different optical devices.

In this embodiment, the waveguide includes a functional portion 94 inwhich the cores 91 must have a smaller width of 4 μm and a refractiveindex of 1.4799, whereas the fibres 93 in a typical fiber array 92 havea greater diameter of 8 μm and are composed of a material having asmaller refractive index. In the embodiment, each of the cores 91terminates by a respective waveguide 95 through a transition region 96of the device 90 which is to be coupled to the device 92. Each waveguide95 is of a different material having a refractive index of 1.4632 and awidth of 7.5 μm, which is matched to that of the fibres 93.

At a transition region 96 of the device 90, core width is graduallyspread from 4 μm to 7.5 μm. FIG. 10 is a cross-sectional view of part ofthe device 90 shown in FIG. 9 (in FIG. 10 the view is in a directionparallel to the surface of the device 90), showing a possible structureof the transition between the cores in the functional portion 91 and inthe coupling waveguide 95. Both are covered by a cladding layer 97. Thisstructure may be achieved by the methods described above.

Because of this, the coupling loss with the optical fibre array 92 isvery much reduced, and the miniaturisation of the waveguide devicebecame possible.

The application of this embodiment is not limited to coupling to aconventional optical fibre. Instead, the embodiment may be adapted tosuit the mode field diameter of the output waveguide 93 of any otheroptical device 92.

All of the devices 20, 30, 40, 50, 60, 70, 90 are preferably formed bythe method of the invention described above in relation to FIG. 1. Inparticular, indeed it is considered that this is presently the onlycommercially realistic way in which they can be formed, the invention isnot limited in this respect.

Although preferred embodiments of the invention have been describedabove, many variations are possible within the scope of the invention aswill be clear to a skilled reader. For example, although all the devicesshown have only two different core materials in different regions, theinvention is not limited in this respect and devices according to theinvention may include core regions formed of any number of differentrespective core materials.

1. A method of producing an optical device, the method including thesteps of: forming trenches within a cladding layer; covering certainareas of the trenches with a cover comprising a photoresist material,wherein the cover is formed by a liftoff process; depositing a firstcore layer of a first core material to fill the trenches which are notcovered; removing the cover; depositing a second core layer of a secondcore material to fill the trenches which were previously covered;removing the first and second core materials outside the trenches in asingle process step; and applying a cladding layer to cover thetrenches.
 2. A method according to claim 1 in which the core materialoutside the trenches is removed by polishing.
 3. An optical devicecomprising at least two optical paths of different geometrical lengths,light passing along the two optical paths interacting, characterised inthat each of the optical paths is being formed as a trench within acladding layer including at least one optical path comprising a firstand second region composed of different core materials and havingdifferent refractive index variation with temperature, the corematerials in the first and second region chosen such that theinteraction between the light passing along the paths is temperatureindependent.
 4. An optical device according to claim 3 which is aninterferometer.
 5. An optical device according to claim 3 which is anarrayed-waveguide grating having a plurality of optical paths.
 6. Anoptical device according to claim 5, each path intersecting with aregion within which the cores are of a different material from the coresoutside the region, the intersection of the optical paths with theregion varying with the geometrical length of the optical paths.
 7. Anoptical device according to claim 3 which is produced by a methodaccording to claim
 1. 8. An optical device according to claim 3 which isproduced by a method according to claim 2.